A Soft Spectrum Filter to Improve the Dosimetric and Biologic Aspects of Flattening Filter Free TrueBeam

Abstract

Design a Soft-Spectrum-Filter (SPECTER) and improve the dosimetric and biologic aspects of Flattening-Filter-Free TrueBeam by reducing the dose to organ-at-risk (OAR) while keeping the high dose rate. In the first part, clinical comparison was made to investigate the dosimetric and biological differences between the flattened beam and the FFF beam. In the second part, a new filter was designed based on the Monte-Carlo (MC) simulation. Varian TrueBeam system was commissioned on the Eclipse. Photon beams of 6 MV and 10 MV were studied to plan fourteen clinical cases. Static intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) were compared. A Matlab code was used to calculate the dose-volume-histogram (DVH), biological effective dose (BED), and equivalent uniform dose (EUD) based on the DICOM files exported from Eclipse. Based on Gay and Niemierko's model, tumor control probability (TCP) and normal tissue complication probability (NTCP) were calculated. For the MC simulation, IAEA phase space data generated by the VirtuaLinac system was used to simulate the beam spectrum of the TrueBeam system. BEAMnrc and DOSXYZnrc were used to model the SPECTER and calculate the dose. Simulation results were benchmarked with the clinical measurement data. SPECTER was designed using the EGSnrc coding system. Comparative target coverage was obtained from the FFF beam as the flattened beam. For OARs with high radiation sensitivity and low dose (e.g. lens of the eye), the FFF beam had up to 38% reduction in mean dose and 34% reduction in maximum dose compared with the flattened beam, leading to 85% reduction in the NTCP value. For OARs which were closer to the radiation field and received higher dose (e.g. brainstem), the FFF beam had up to 14% mean dose reduction, 10% maximum dose reduction and 55% reduction in the NTCP value. For the MC simulation, lead SPECTER with circular cross section provided the best dose attenuation in the out-of-field region. Compared with FFF beam, the SPECTER beam provided 3% reduction for the total dose and 2.5% reduction for the internal scatter dose for 25x25 cm2 field size in the water tank at d = 10 cm depth. FFF beam could provide similar target coverage as the flattened beam in all 14 clinical cases. Significant dose sparing effect of the FFF beam was observed for the head and neck cases with relative large treatment field (e.g. 16x20 cm2). Due to the speed limitation of the MLC, the maximum dose rate of the FFF beam may not always be achievable for large treatment field size (e.g. 20x20 cm2).A new filter is proposed to further reduce the dose in the tail region of the FFF beam in order to improve the dose sparing effect of the FFF beam. The geometry of the SPECTER enables us to keep the high dose rate of the FFF beam.